Expansion of the invasive Balkan slug Tandonia kusceri (Stylommatophora: Milacidae): A new frontier in northern Ukraine and other new records

The distribution of the invasive slug Tandonia kusceri is reviewed and mapped with our new findings and previously unpublished recent reports based on photographs from citizen science databases (iNaturalist, UkrBIN). Two new colonies of T. kusceri were found and monitored in Kyiv city (Northern Ukraine) during the 2021–2022. These colonies are about 350 km north from the nearest previously published records of this species, as well as any other species of Milacidae in Eastern Europe. The slugs inhabit the urban open and semi-open landscapes of Kyiv city together with several other invasive molluscs, in one locality notably together with Oxychilus translucidus, Harmozica ravergiensis and Xerolenta obvia. One specimen of T. kusceri was found in a basement of a private estate in Radyvyliv town (Rivne region, Northern Ukraine) in 2020, but it is unknown whether it represents an established colony. Previously unpublished reports of T. kusceri on iNaturalist include two reliable findings from the Caucasian region, in Georgia and Russia, both in 2021. These are the first reports of any Tandonia in Caucasus, as well as in the mentioned countries, with one in Georgia being the eastmost frontier for this genus. In addition to a previously published 2013 report of T. kusceri near Chicago, USA, there are multiple iNaturalist reports of this slug in southern Ontario, Canada (600 km east of Chicago), starting from 2017. Moreover, there are several new reports of T. kusceri in Ukraine, including Kherson, Dnipro, Kryvyi Rig, Marhanets and Kamianets-Podilskyi cities. Considering that most of our new T. kusceri records are from the 2020s, it is most likely that slug has invaded these regions very recently and is still continuing to expand northward and eastward in Europe. This species is expected to further extend its range both in Europe and North America.